Antarctic sea ice elevation from satellite radar altimetry

In situ measurements of sea ice thickness from ship and upward-looking sonar are used to assess the potential for satellite radar altimetry to provide information on Antarctic sea ice thickness. A climatology of satellite ice elevation estimates is compared to an Antarctic sea ice thickness climatology made from the Antarctic Sea Ice Processes and Climate ( ASPeCt) data set. In addition monthly, regional, satellite ice elevation estimates are compared to ULS ice draft data. The results show reasonable spatial agreement between the satellite and in-situ data, and show regional signals of change in ice elevation in line with that which would be expected. The results show some promise for providing information on Antarctic ice thickness from radar altimetry missions such as CryoSat. However, further studies into snow and ice density and the radar penetration into the Antarctic snow cover are required

Circumpolar thinning of Arctic sea ice following the 2007 record ice extent minimum

September 2007 marked a record minimum in sea ice extent. While there have been many studies published recently describing the minimum and its causes, little is known about how the ice thickness has changed in the run up to, and following, the summer of 2007. Using satellite radar altimetry data, covering the Arctic Ocean up to 81.5 degrees North, we show that the average winter sea ice thickness anomaly, after the melt season of 2007, was 0.26 m below the 2002/2003 to 2007/2008 average. More strikingly, the Western Arctic anomaly was 0.49 m below the six-year mean in the winter of 2007/2008. These results show no evidence of short-term preconditioning through ice thinning between 2002 and 2007 but show that, after the record minimum ice extent in 2007, the average ice thickness was reduced, particularly in the Western Arctic. Citation: Giles, K. A., S. W. Laxon, and A. L. Ridout (2008), Circumpolar thinning of Arctic sea ice following the 2007 record ice extent minimum, Geophys. Res. Lett., 35, L22502, doi: 10.1029/2008GL035710

Western Arctic Ocean freshwater storage increased by wind-driven spin-up of the Beaufort Gyre

The Arctic Ocean’s freshwater budget comprises contributions from river runoff, precipitation, evaporation, sea-ice and exchanges with the North Pacific and Atlantic. More than 70,000km3 of freshwater are stored in the upper layer of the Arctic Ocean, leading to low salinities in upper-layer Arctic sea water, separated by a strong halocline from warm, saline water beneath. Spatially and temporally limited observations show that the Arctic Ocean’s freshwater content has increased over the past few decades, predominantly in the west. Models suggest that wind-driven convergence drives freshwater accumulation. Here we use continuous satellite measurements between 1995 and 2010 to show that the dome in sea surface height associated with the western Arctic Beaufort Gyre has been steepening, indicating spin-up of the gyre. We find that the trend in wind field curl—a measure of spatial gradients in the wind that lead to water convergence or divergence—exhibits a corresponding spatial pattern, suggesting that wind-driven convergence controls freshwater variability. We estimate an increase in freshwater storage of 8,000±2,000km3 in the western Arctic Ocean, in line with hydrographic observations, and conclude that a reversal in the wind field could lead to a spin-down of the Beaufort Gyre, and release of this freshwater to the Arctic Ocean

Tracer-derived freshwater composition of the Siberian continental shelf and slope following the extreme Arctic summer of 2007

We investigate the freshwater composition of the shelf and slope of the Arctic Ocean north of the New Siberian Islands using geochemical tracer data (delta O-18, Ba, and PO4*) collected following the extreme summer of 2007. We find that the anomalous wind patterns that partly explained the sea ice minimum at this time also led to significant quantities of Pacific-derived surface water in the westernmost part of the Makarov Basin. We also find larger quantities of meteoric water near Lomonosov Ridge than were found in 1995. Dissolved barium is depleted in the upper layers in one region of our study area, probably as a result of biological activity in open waters. Increasingly ice-free conditions compromise the quantitative use of barium as a tracer of river water in the Arctic Ocean. Citation: Abrahamsen, E. P., M. P. Meredith, K. K. Falkner, S. Torres-Valdes, M. J. Leng, M. B. Alkire, S. Bacon, S. W. Laxon, I. Polyakov, and V. Ivanov (2009), Tracer-derived freshwater composition of the Siberian continental shelf and slope following the extreme Arctic summer of 2007, Geophys. Res. Lett., 36, L07602, doi:10.1029/2009GL037341

Classification of CryoSat-2 Radar Echoes

Sea-ice thickness at global scale is an important variable in the polar climate system. Only satellite altimeters such as onboard the CryoSat-2 mission allow us to obtain sea-ice thickness on hemispherical scale. Accurate CryoSat-2 altimeter range measurements provide surface elevations which have to be referenced to the local sea level to obtain sea-ice freeboard that can be converted into sea-ice thickness assuming hydrostatic equilibrium. The local sea-surface height is determined by careful detection of leads in the ice surface using the specific characteristics of the radar signal. Off-nadir reflections from leads can significantly affect the range retracking and hence bias the surface elevations of leads and sea ice. This can finally lead to a negative freeboard and hence also affects the thickness and volume retrieval. We present a method for the classification of CryoSat-2 radar echoes to correctly discriminate between valid and off-nadir biased echoes. We apply our classification to a CryoSat-2 track from December 15 where 50 leads over a distance of 2,300 km are identified. Overall 22 % of the surface elevations are associated with biased radar echoes